From Halopedia, the Halo wiki
Character interludes are a storytelling aspect of the Halo 3 campaign, in which either Cortana or the Gravemind telepathically address the Master Chief and often muse introspectively. During these interludes, the player's movements will slow drastically, as will their ability to turn and face another direction. A screen effect will also alter the display of the game. Referred to as "Cortana channel" and "Cortana effect" by Bungie employees, the character interludes is an attempt at weaving story into gameplay sequences without affecting the pacing.
During a Cortana interlude, the screen turns blue, and Cortana's image flashes in and out of focus on the screen. Her voice contains a slight echo and sometimes a deeply-pitched distortion effect, and her image erratically "jumps" around the screen. During a Cortana interlude there will also be a noise sounding vaguely similar to the noise heard if one places a seashell in their ear.
During a Gravemind interlude, the player's view become murky green, and tentacle-like imagery can be seen at the edge of the screen. The screen will shake in unison with the Gravemind's voice, shaking far more violently if Gravemind is screaming or yelling in anger at John-117.
The Cortana interludes depict her descent into rampancy, while the Gravemind interludes are used to convey the point of view of the Flood. It is possible in co-op mode for one player to trigger the interlude while the other player is behind fighting enemies that the first one skipped. During any of these interludes, the player is completely immune to damage and will not be attacked even by nearby enemies.
The Halo Evolutions story Human Weakness revealed that the Gravemind had successfully hijacked Cortana's system, allowing him to speak with her voice. This was part of his plan to persuade Cortana and John-117 that Cortana was descending into rampancy. Cortana battled the Gravemind, trying to hold onto the data about the Portal and the Ark. She ends up creating multiple copies of herself to distract the Gravemind and twice they apparently chime in during her conversations with the Gravemind. One adds the "I am a monument to all your sins" part of one of the interludes while the rest is Cortana defying Gravemind to buy the Master Chief time. Cortana places the message onto Indulgence of Conviction by composing it with the help of a copy and then transmitting it to that ship through Gravemind's link to it, with part of it being coded to seek out and upload itself to the nearest data device. Gravemind then reveals that he knew and helped out as he wanted to lure John-117 to the Flood-infested High Charity, but did not know what the message actually said.
The ninth Halo Legendary Crate Data Drop discusses the "Cortana moments" further, revealing that John-117 did recieve the messages throughout the later weeks of 2552, believing them to be software glitches and anomalies. His MJOLNIR armor was later checked out by a trauma analysis team, with the Cortana transmissions officially ruled to be "residual interface anomalies"; echoes of Cortana's personality matrix left inside the armor systems after her departure. After reuniting with Cortana, John never discussed the transmissions, and later in 2553 his armor filed a Software Trauma Analysis detailing the supposed-glitches. Following Cortana's sacrifice during the New Phoenix Incident, John-117 was debriefed by the UNSC, with the Cortana transmissions a subject of this discussion, with analysts pointing out that Cortana's ability to communicate across thousands of light years may present a vital security flaw in UNSC systems - a shadow of events to come.
List of Interludes